Modulation system and method for outputting high-power continuous single longitudinal mode laser

Abstract

The invention discloses a modulation system and method for outputting high-power continuous single longitudinal mode laser. The modulation system comprises an LD pumping source, a coupling lens and an NPRO crystal. Pumping laser emitted by the LD pumping source passes through the coupling lens, then is incident at an angle of 45 degrees with the normal of the incident plane of the NPRO crystal, is reflected by the total emission plane, and then is emergent at an angle of 45 degrees with the normal of the incident plane of the NPRO crystal, so that a non-planar annular cavity structure is formed; and carrying out coating design on the incident plane of the NPRO crystal. The whole structure is simple and easy to adjust, no extra insertion loss exists, the light path adjusting difficulty is reduced, the NPRO crystal is accurately coated, the loss can be further reduced, and the output power is improved compared with other methods.

Description

Technical Field

[0001] The invention belongs to the technical field of laser research, and in particular relates to a modulation system and method for outputting high-power continuous single longitudinal mode laser. Background Art

[0002] Regarding the modulation method of high-power continuous single longitudinal mode laser, the existing technologies at home and abroad can be divided into the following two types: one is to obtain 1116nm laser method through distributed feedback semiconductor laser, and the other is to directly output 1116nm laser method by forming an oscillator through cavity mirror and Nd:YAG laser crystal and inserting frequency selection device.

[0003] The structure of the 1116nm laser method based on distributed feedback semiconductor laser is as follows Figure 1 As shown. This method is to obtain the required 1116nm laser through a distributed feedback semiconductor laser and amplify it using an ytterbium-doped fiber amplifier (YDFA). One of the most effective methods for a distributed feedback semiconductor laser to achieve single longitudinal mode operation is to build a Bragg grating inside it. Figure 2 As shown, the longitudinal mode selection is realized by relying on the frequency selection principle of the grating. At present, the entire working system of the 1116nm laser method based on distributed feedback semiconductor laser is large in size and expensive, and is not used in scientific research and production applications.

[0004] Based on the cavity mirror and Nd:YAG laser crystal to form an oscillator and insert a frequency selection device to output 1116nm laser method Figure 3 As shown. This method uses a cavity mirror and a Nd:YAG laser crystal to form an oscillator and inserts a frequency-selective device, the Fabry-Perot etalon, to output a 1116nm laser. This method has a mode matching problem, which will be more difficult in subsequent experimental operations. In addition, the insertion of a frequency-selective device will introduce some insertion loss, which will reduce the working efficiency of the entire system and the output power.

[0005] Based on the above problems in the prior art, it is urgent to propose a modulation system and method for outputting high-power continuous single longitudinal mode laser, optimize the overall structure of the system, and improve work efficiency. Summary of the invention

[0006] In order to solve the above technical problems, the present invention proposes a modulation system and method for outputting high-power continuous single-longitudinal mode laser, which does not require the insertion of redundant frequency selection devices, has a compact and efficient overall structure, high working efficiency and relatively high output power, simple optical path alignment and relatively concentrated spectrum energy, so as to solve the problems existing in the above-mentioned prior art.

[0007] To achieve the above object, the present invention provides a modulation system for outputting high-power continuous single longitudinal mode laser, comprising: an LD pump source, a coupling lens and an NPRO crystal;

[0008] The pump laser emitted by the LD pump source passes through the coupling lens and is incident at 45° to the normal of the incident surface of the NPRO crystal, and is emitted at 45° to the normal of the incident surface of the NPRO crystal after being reflected by the total emission surface, thereby forming a non-planar annular cavity structure;

[0009] The incident surface of the NPRO crystal is designed for coating.

[0010] Optionally, the NPRO crystal is made of Nd 3+ Nd:YAG crystal with a doping concentration of 0.8at%;

[0011] The refractive index of the NPRO crystal is 1.82, and the Verdet constant is 103°T -1 m -1 The crystal size is 12mm×10mm×3mm, the included angle is 25.21°, and the dihedral angle is 45.12°.

[0012] Optionally, the incident surface and the exit surface of the NPRO crystal are the same surface, and the coating parameters of the incident surface are 808nm high-transmittance film.

[0013] The present invention also provides a modulation method for outputting high-power continuous single longitudinal mode laser, based on a modulation system for outputting high-power continuous single longitudinal mode laser, comprising the following steps:

[0014] The pump laser is incident at 45° to the normal of the incident surface of the NPRO crystal and propagates in a ring in the non-planar ring cavity structure;

[0015] Obtain the Jones matrix change, change matrix and polarization change matrix generated by reflection in circular propagation;

[0016] Based on the Jones matrix change, the change matrix and the polarization change matrix, a propagation matrix of the pump laser is obtained;

[0017] Based on the propagation matrix, the polarization state of the pump laser remains unchanged after one round of circular propagation, and a high-power continuous single longitudinal mode laser is output.

[0018] Optionally, four points are selected from the NPRO crystal, point A is located on the incident surface, and points B, C, and D are located on the total reflection surface respectively;

[0019] The matrix corresponding to the Jones matrix change is:

[0020]

[0021] B→C:J2=R(θ BC ), C→D: J3=R(-θ BC ), D→A:J4=R(θAB )

[0022] in, is the coordinate system change matrix, J1 is the Jones matrix change from A to B, J2 is the Jones matrix change from B to C, J3 is the Jones matrix change from C to D, and J4 is the Jones matrix change from D to A.

[0023] Optionally, the change matrix is:

[0024] J D =J B

[0025] Among them, r p is the amplitude reflection coefficient when the polarization direction is parallel to the incident plane, r s is the amplitude reflection coefficient when the polarization direction is perpendicular to the incident plane, i represents the imaginary unit, Δ B is the change in phase difference caused by total reflection of light at point B, Δ C is the change in phase difference caused by total reflection of light at point C, J A is the change matrix of point A, J B is the change matrix of point B, J C is the change matrix of point C, J D is the change matrix of point D.

[0026] Optionally, the polarization change matrix is:

[0027] A→B:J ABV =R(θ V1 ),B→C:J BCV =R(θ V2 ),B→A:J BAV =R(-θ V1 ), C→B:J CBV =R(-θ V2 )

[0028] Among them, point E is located at the midpoint of points BD in the plane where points ABD of the crystal are located, θ V1 is the magnetic declination of the AE segment, θ V2 is the magnetic declination of the CE segment, J ABV is the A→B polarization change matrix, J BCV is the B→C polarization change matrix, J BAV is the C→D polarization change matrix, J CBV is the D→A polarization change matrix.

[0029] Optionally, the propagation matrix is:

[0030] J CCW =J A J4J BAV ·JD J3J CBV ·J C J2J BCV ·J B J1J ABV .

[0031] Compared with the prior art, the present invention has the following advantages and technical effects:

[0032] The modulation system proposed by the present invention includes: an LD pump source, a coupling lens and an NPRO crystal; the pump laser emitted by the LD pump source passes through the coupling lens and is incident at 45° to the normal of the incident surface of the NPRO crystal, and is emitted at 45° to the normal of the incident surface of the NPRO crystal after being reflected by the full emission surface, thereby forming a non-planar annular cavity. It can be seen that the overall structure is simple and easy to adjust, and there is no additional insertion loss, the difficulty of optical path adjustment is reduced, and the work efficiency is greatly improved.

[0033] The present invention also precisely coats the NPRO crystal, which can further reduce the loss and improve the output power compared with other methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0035] Figure 1 A structural diagram of a method for obtaining 1116nm laser light based on a distributed feedback semiconductor laser in the background technology of the present invention;

[0036] Figure 2 The Bragg grating diagram in the background technology of the present invention;

[0037] Figure 3 It is a structural diagram of an oscillator composed of a cavity mirror and a Nd:YAG laser crystal and inserted with a frequency selection device in the background technology of the present invention;

[0038] Figure 4 This is a structural diagram of a modulation system for outputting high-power continuous single longitudinal mode laser according to an embodiment of the present invention;

[0039] Figure 5 The NPRO crystal structure diagram of an embodiment of the present invention;

[0040] Figure 6 Schematic diagram of the NPRO crystal laser path according to an embodiment of the present invention. DETAILED DESCRIPTION

[0041] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0042] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0043] Embodiment 1

[0044] like Figure 4 As shown, in this embodiment, a modulation system for outputting high-power continuous single longitudinal mode laser is provided, including: an LD pump source, a coupling lens and an NPRO crystal; the pump laser emitted by the LD pump source passes through the coupling lens and is incident at 45° to the normal of the incident surface of the NPRO crystal, and is emitted at 45° to the normal of the incident surface of the NPRO crystal after being reflected by the total emission surface, thereby forming a non-planar annular cavity structure.

[0045] Furthermore, a coating design is performed on the incident surface of the NPRO crystal.

[0046] Furthermore, the LD pump source emits a laser with a wavelength of 808nm, which is transmitted to the incident surface of the NPRO crystal through the coupling lens system, oscillates in a ring inside the crystal for one circle, and finally outputs a 1116nm laser under the action of the magnetic field.

[0047] Furthermore, NPRO crystal is based on Nd 3+ A monolithic non-planar ring cavity structure with a Nd:YAG crystal doped with a concentration of 0.8 at% as the gain medium has a refractive index of 1.82 and a Verdet constant of V e =103°T -1 m -1 , the crystal size is 12mm×10mm×3mm, the angle θ is 25.21°, and the dihedral angle β is 45.12°. Figure 5 As shown in the figure, the planes where the four points A, B, C, and D are located are polished, and the other surfaces are matte. The plane where point A is located is both the incident plane of the LD pump laser and the output plane of the oscillating laser. The three planes where points B, C, and D are located are all total reflection planes. The light oscillates along the direction A→B→C→D in the crystal. The pump laser is incident through the plane where point A is located. The incident pump light is 45° to the normal of the plane where point A is located. Then, after passing through the NPRO crystal, the outgoing laser is 45° to the normal of the plane where point A is located, thus forming a non-planar ring cavity structure.

[0048] The NPRO crystal used in this embodiment is first designed in terms of size and angle. The size and angle are designed to reduce the astigmatism of the entire resonant cavity. Then the focus is on the precise coating design on the plane where point A is located. The transmittance of the 808nm pump light is improved by calculating and designing the transmittance of the pump light and the oscillating light, so that the 808nm pump light can enter the crystal better. The transmittance of the 1064nm laser is precisely designed to suppress the oscillation of the 1064nm laser, thereby ensuring the output of high-power continuous single longitudinal mode 1116nm laser. The non-planar ring cavity uses the phase delay of light on the total reflection surface with different spatial orientations in the cavity and the Faraday rotation effect brought by the external magnetic field to make the forward and reverse propagating light waves have different intrinsic polarization states and eigenvalues. By coating on the output surface, polarized light in different directions has different reflection coefficients. Finally, only a certain polarization state of a certain rotation direction can be oscillated to achieve unidirectional traveling wave oscillation.

[0049] This embodiment also provides a modulation method for outputting high-power continuous single longitudinal mode laser, based on a modulation system for outputting high-power continuous single longitudinal mode laser, comprising the following steps:

[0050] The pump laser is incident at 45° to the normal of the incident surface of the NPRO crystal and propagates in a ring in the non-planar ring cavity structure;

[0051] Obtain the Jones matrix change, change matrix and polarization change matrix generated by reflection in circular propagation;

[0052] Based on the Jones matrix change, the change matrix and the polarization change matrix, a propagation matrix of the pump laser is obtained;

[0053] Based on the propagation matrix, the polarization state of the pump laser remains unchanged after one round of circular propagation, and a high-power continuous single longitudinal mode laser is output.

[0054] As a specific embodiment, in the annular optical path, the incident light at the starting position is represented by the Jones matrix as follows: x indicates that the polarization direction is parallel to the incident plane, y indicates that the polarization direction is perpendicular to the incident plane, E p represents the electric field component whose polarization direction is parallel to the incident plane, E s Represents the electric field component whose polarization direction is perpendicular to the incident plane. The following factors lead to the change of polarization matrix during circular propagation:

[0055] The change of the Jones matrix caused by the change of the coordinate system is: There are four places in the circular propagation process.

[0056]

[0057] B→C:J2=R(θ BC ), C→D: J3=R(-θBC ), D→A:J4=R(θ AB ).

[0058] Jones matrix changes caused by reflection: light is totally reflected at points B, C, and D, and the phase difference Δ = φ P -φ S ,φ p is the phase when the polarization direction is parallel to the incident plane, φ s is the phase when the polarization direction is perpendicular to the incident plane.

[0059] According to Fresnel's formula, Where: θ j is the incident angle, n is the refractive index of the medium, -π<Δ≤π. Then the change matrix generated by reflection is:

[0060]

[0061] Among them, r p is the amplitude reflection coefficient when the polarization direction is parallel to the incident plane, r s is the amplitude reflection coefficient with the polarization direction perpendicular to the incident plane, i represents the imaginary unit, and the imaginary unit "i" is usually related to the phase change. Since the phase of light may change when passing through certain optical elements, this change can be represented by introducing the imaginary part in the elements of the Jones matrix (i.e. multiplying by "i"). In this way, the Jones matrix can simultaneously describe the amplitude and phase changes of the light field, thereby achieving a complete description of the polarization state of the light field. B is the change in phase difference caused by total reflection of light at point B, Δ C is the change in phase difference caused by total reflection of light at point C, J A is the change matrix of point A, J B is the change matrix of point B, J C is the change matrix of point C, J D is the change matrix of point D.

[0062] like Figure 6 As shown in the figure, due to the existence of a magnetic field parallel to the AE direction in space, the Faraday rotation effect will cause a certain change in the polarization state of light. The magnetic declination angle θ of the AE segment V1 =V e ·B·AE, CE segment magnetic declination is θ V2 =V e ·B·CE·cosβ. V e is the Verdet constant in the gain medium, B is the magnetic field intensity, then the polarization change matrix J caused by A→B ABV =R(θ V1 ), similarly: B→C:J BCV =R(θ V2 ),B→A:JBAV =R(-θ V1 ), C→B:J CBV =R(-θ V2 ).

[0063] When the light oscillates counterclockwise in the cavity for one cycle, its propagation matrix is:

[0064] J CCW =J A J4J BAV ·J D J3J CBV ·J C J2J BCV ·J B J1J ABV .

[0065] It is required that the polarization state remains unchanged after the light ring rotates one circle, so as to achieve the purpose of intracavity oscillation, that is:

[0066]

[0067] The modulus |λ represents the ratio of the amplitude of the intrinsic polarized light after one circle to the original amplitude when the gain is ignored, 1-|λ 2 Indicated by R S , R P The loss caused by not being equal to 1. S is the reflectivity when the polarization direction is perpendicular to the incident plane, R P is the reflectivity when the polarization direction is parallel to the incident plane.

[0068] When other conditions are the same, if the loss difference of each eigenvalue is greater than 0.01% as stated by ARClobes, only the mode with the smallest loss can be oscillated first and suppress the oscillation of other modes in the average broadened gain medium. The traveling wave oscillation of the ring cavity eliminates the spatial hole burning of the gain and ensures the single longitudinal mode output. In order to meet the single-mode oscillation, it is calculated that the applied magnetic field is 0.5T, the incident light is irradiated at an incident angle of 45°, the coating parameters of the output surface are 808nm high-transmittance film, the transmittance of 1064nm is 30%, and the transmittance in different polarization directions of 1116nm is different, R S 97.4%, R P is 85%.

[0069] In this embodiment, high-power continuous single longitudinal mode 1116nm laser output can be achieved by combining an LD pump source and an NPRO crystal and precisely coating the NPRO crystal.

[0070] From the structure of the modulation system proposed in this embodiment, the overall structure is simple and easy to adjust, and there is no additional insertion loss, the difficulty of optical path adjustment is reduced, and precise coating of the NPRO crystal can further reduce the loss and improve the output power compared with other methods. In addition, this solution can also perform frequency doubling design on the output 1116nm laser to output high-power continuous single longitudinal mode 558nm laser.

[0071] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A modulation system for outputting high-power continuous single longitudinal mode laser, characterized in that: include: LD pump source, coupling lens and NPRO crystal; The pump laser emitted by the LD pump source passes through the coupling lens and is incident at 45° to the normal of the incident surface of the NPRO crystal, and is emitted at 45° to the normal of the incident surface of the NPRO crystal after being reflected by the total emission surface, thereby forming a non-planar annular cavity structure; The incident surface of the NPRO crystal is designed for coating.

2. The modulation system for outputting high-power continuous single longitudinal mode laser according to claim 1, characterized in that: The NPRO crystal is made of Nd 3+ Nd:YAG crystal with a doping concentration of 0.8at%; The refractive index of the NPRO crystal is 1.82, and the Verdet constant is 103°T -1 m -1 The crystal size is 12mm×10mm×3mm, the included angle is 25.21°, and the dihedral angle is 45.12°.

3. The modulation system for outputting high-power continuous single longitudinal mode laser according to claim 1, characterized in that: The incident surface and the exit surface of the NPRO crystal are the same surface, and the coating parameters of the incident surface are 808nm high-transmittance film.

4. A modulation method for outputting high-power continuous single longitudinal mode laser, characterized in that: The modulation system for outputting high-power continuous single longitudinal mode laser according to any one of claims 1 to 3 comprises the following steps: The pump laser is incident at 45° to the normal of the incident surface of the NPRO crystal and propagates in a ring in the non-planar ring cavity structure; Obtain the Jones matrix change, change matrix and polarization change matrix generated by reflection in circular propagation; Based on the Jones matrix change, the change matrix and the polarization change matrix, a propagation matrix of the pump laser is obtained; Based on the propagation matrix, the polarization state of the pump laser remains unchanged after one round of circular propagation, and a high-power continuous single longitudinal mode laser is output.

5. The modulation method for outputting high-power continuous single longitudinal mode laser according to claim 4, characterized in that: Select four points from the NPRO crystal, point A is located on the incident surface, and points B, C, and D are located on the total reflection surface respectively; The matrix corresponding to the Jones matrix change is: B→C:J2=R(θ BC ), C→D:J3=R(-θ BC ), D→A:J4=R(θ AB ) in, is the coordinate system change matrix, J1 is the Jones matrix change from A to B, J2 is the Jones matrix change from B to C, J3 is the Jones matrix change from C to D, and J4 is the Jones matrix change from D to A.

6. The modulation method for outputting high-power continuous single longitudinal mode laser according to claim 5, characterized in that: The change matrix is: I D =J B Among them, r p is the amplitude reflection coefficient when the polarization direction is parallel to the incident plane, r s is the amplitude reflection coefficient when the polarization direction is perpendicular to the incident plane, i represents the imaginary unit, Δ B is the change in phase difference caused by total reflection of light at point B, Δ C is the change in phase difference caused by total reflection of light at point C, J A is the change matrix of point A, J B is the change matrix of point B, J C is the change matrix of point C, J D is the change matrix of point D.

7. The modulation method for outputting high-power continuous single longitudinal mode laser according to claim 6, characterized in that: The polarization change matrix is: A→B:J ABV =R(θ V1 ),B→C:J BCV =R(θ V2 ),B→A:J BAV =R(-θ V1 ),C→B:J CBV =R(-θ V2 ) Among them, point E is located at the midpoint of points BD in the plane where points ABD of the NPRO crystal are located, θ V1 is the magnetic declination of the AE segment, θ V2 is the magnetic declination of the CE segment, J ABV is the A→B polarization change matrix, J BCV is the B→C polarization change matrix, J BAV is the C→D polarization change matrix, J CBV is the D→A polarization change matrix.

8. The modulation method for outputting high-power continuous single longitudinal mode laser according to claim 7, characterized in that: The propagation matrix is: <h2 style=";text-align:left;direction:ltr">J<h2 style=";text-align:left;direction:ltr"> CCW <h2 style=";text-align:left;direction:ltr"> =J<h2 style=";text-align:left;direction:ltr"> A <h2 style=";text-align:left;direction:ltr"> J4J<h2 style=";text-align:left;direction:ltr"> BAV <h2 style=";text-align:left;direction:ltr"> ·J<h2 style=";text-align:left;direction:ltr"> D <h2 style=";text-align:left;direction:ltr"> J3J<h2 style=";text-align:left;direction:ltr"> CBV <h2 style=";text-align:left;direction:ltr"> ·J<h2 style=";text-align:left;direction:ltr"> C <h2 style=";text-align:left;direction:ltr"> J2J<h2 style=";text-align:left;direction:ltr"> BCV <h2 style=";text-align:left;direction:ltr"> ·J<h2 style=";text-align:left;direction:ltr"> B <h2 style=";text-align:left;direction:ltr"> J1J<h2 style=";text-align:left;direction:ltr"> ABV <h2 style=";text-align:left;direction:ltr"> 。

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